Threaded fasteners typically consist of a nut and a bolt. The nut has an internal thread that screws onto an external thread of the bolt. Wrenching surfaces on the nut and/or bolt accept wrenches that tightly join the fasteners and one or more workpieces together. The fastener secures workpieces together to form a joint by compressive engagement between the nut on one side of the workpiece, and the head of the bolt on the opposite side.
U.S. Pat. No. 4,260,005 describes a particular type of threaded fastener having a self-locking nut that uses external lobes to accept a driver having a generally triangular or deltoid shaped socket to tighten the nut on a cooperating pin. The teachings of this patent necessary for an understanding of the present invention are incorporated herein by this reference. The nut is preferably used with a pin having a plurality of grooves or flutes extending longitudinally along the pin. Once a predetermined axial load exists in the joint being made, the lobes plastically deform and the driver can no longer turn the nut. Deformation of the lobes displaces material positioned radially inward from them into the flutes of the cooperating pin to produce a mechanical thread lock. The thread lock results from material deforming into and across the pin flutes.
There are numerous applications where it is advantageous to provide a fastener which may be held against rotation from its leading edge while a nut is torqued upon the pin. For example, as is often the case in the aerospace industry, there are many requirements for "blind" bolt applications, where the head of the bolt cannot be conveniently reached, or is perhaps altogether unreachable for wrenching purposes. For these applications, a holding bit or tool is often provided with the driver. The bit engages a recess in the threaded end of the pin to facilitate application of the nut by the driver. The bit remains stationary, thus holding the pin against rotation, while the driver turns to torque the nut. U.S. Pat. Nos. 4,583,483 and 5,044,225 describe the use of such a bit or tool in connection with power tools developed for the rapid installation of fasteners.
Generally, a hexagonal recess is provided in the threaded end of bolts for receiving a matching hexagonal bit. Existing hexagonal bits, however, have experienced unacceptable failure rates when used during installation of bolts in certain wet-sealant applications. In these applications, sealant often surrounds the head of the bolt and is present between the bolt and the workpiece. As a result, all of the torque required to apply the nut to the pin is transferred directly to the bit, rather than being partially absorbed by the friction normally present between the head of the bolt and the workpiece. When the above induced torque exceeds the strength of the hexagonal bit, application of the nut becomes impossible.
Initial attempts at solving this problem included varying the cross-section and material of the hexagonal bit. However, these attempts have been largely unsuccessful due to radial loading for hexagonal recesses. Spline keys and drivers are known and have been used in many applications. When used in connection with fasteners, standard spline recesses and wrenching tools result in the splines in the fastener being sheared by the tool. Torque is transmitted from the mating spline of the tool to the spline of the recess. When resistance is achieved after seating the fastener, additional torquing can result in the shearing of the splined socket. Shearing results because standard spline design results in the wrenching drive tool being fabricated from a material that is usually stronger than the material of the fastener. Standard configurations results in the spline surface area of the wrenching tool being 7.9% to 38.5% greater than the spline surface area of the fastener socket's spline surface area. This coupled with the tool having a greater shear strength than the fastener's socket results in an unacceptable failure rate.
Consequently, a need exists for a spline drive socket/tool arrangement for fasteners that prevent unacceptable failure.